Clearance and micropuncture studies were performed to investigate the relationship between sodium and phosphate transport in 10 acutely thyroparathyroidectomized dogs with unilateral remnant kidney before and after expansion of extracellular volume. The remnant kidney was induced by ligation of 3/4 to 5/6 branches of renal artery in one kidney which resulted in a reduction in glomerular filtration rate to about 1/5 of the contralateral kidney. Mean plasma ultrafilterable phosphate was relatively high at 2.49 mmoles/liter due to high dietary intake of phosphate rebasorption was reduced by 21% of the filtered load while that of sodium was reduced by 15% following volume expansion. There were prominent increases in fractional excretion of phosphate and sodium by the two kidneys. Although the increase in fractional sodium excretion was greater in the remnant kidney as compared to the contralateral kidney, the changes in fractional phosphate excretion were similar bilaterally. It was concluded that changes in proximal tubule sodium transport can directly influence phosphate transport by the remnant kidney in the absence of parathyroid glands. However, the slightly different phosphate response by the remnant kidney as compared to that of sodium indicates a possible dissociation of phosphate and sodium transport in the distal nephron.